A general three-phase inverter device smoothes out voltage ripple occurring in a diode bridge by providing a capacitor and a reactor in a smoothing circuit. However, a large capacitor and a large reactor are required to smooth out voltage ripple, which raises a problem of increase in cost and volume.
When the sizes of the capacitor and the reactor are reduced in such an inverter device, the above problem is solved, but a DC voltage pulsates. The pulsation of the DC voltage causes ripple and beat in a motor current.
As illustrated in FIG. 4, it is known that when there is ripple in a DC link voltage, a specific command frequency of an inverter device causes undulation (beat) of low-frequency components in a motor phase current.
In the case of an inverter device not using an electrolytic capacitor for smoothing, the largest frequency component generated in the DC link voltage is six times the power source frequency (300 Hz at 50 Hz, 360 Hz at 60 Hz), and when the number of motor pole pairs is three, the beat phenomenon becomes most prominent around 100 rps (300 Hz) or 120 rps (360 Hz).
Such a beat phenomenon causes problems such as increase in the rms value and peak current, and vibration of the motor. Hence, a technique for suppressing beat is necessary.
A conventional inverter device detects ripple in a DC link voltage and corrects the voltage, so that a voltage according to a voltage command is applied from the inverter device to a motor to suppress beat (for example, refer to JP S61-48356 B. As a result, ripple in the voltage applied to the motor is suppressed, and ripple and beat in the motor current are suppressed.